Paraspinal Muscle Fat-Free Cross-Sectional Area Shows Superior Performance to Vertebral Bone Quality Score for Osteoporosis Identification on Lumbar MRI

Purpose To compare the diagnostic value of the VBQ score with quantitative paraspinal muscle parameters derived from routine lumbar MRI. Methods This retrospective cross-sectional study analysed 150 patients who had undergone both lumbar magnetic resonance imaging (MRI) and dual-energy X-ray absorptiometry (DXA). Osteoporosis was defined as a DXA T-score of ≤ − 2.5. MRI-based measurements included the L1–L4 VBQ score, as well as cross-sectional area (CSA), fat-free cross-sectional area (FCSA), and Dixon fat fraction (DFF) of the erector spinae plus multifidus (ES + MF) and psoas major (PM). Associations were examined using Spearman correlation and logistic regression analyses. Diagnostic performance was evaluated using receiver operating characteristic (ROC) curve analysis. Results Patients with osteoporosis exhibited higher VBQ scores (P = 0.019) and markedly lower CSA and FCSA of both ES + MF and PM (all P < 0.001). The lowest T-score showed a weak inverse correlation with VBQ (ρ = −0.273) but moderate positive correlations with muscle CSA and FCSA, strongest for PM FCSA (ρ = 0.518). In multivariable analyses, only FCSA parameters remained independently associated with osteoporosis, whereas VBQ lost statistical significance. VBQ demonstrated limited discriminative ability (AUC = 0.611), while ES + MF FCSA (AUC = 0.759) and PM FCSA (AUC = 0.761) showed substantially better performance. Combining VBQ with PM FCSA yielded a modest improvement (AUC = 0.785). Conclusion Paraspinal muscle fat-free cross-sectional area, particularly of the psoas major, shows a consistent and independent association with osteoporosis and outperforms the VBQ score alone. Quantitative assessment of functional muscle mass on routine lumbar MRI may support opportunistic osteoporosis screening in patients with degenerative lumbar spine disorders.